None
Not Applicable
The present invention is related to the field of power semiconductor devices, and more particularly to circuits and structures for sensing thermal operating characteristics of power devices for device protection or other purposes.
Thermal protection of integrated power devices, such as integrated MOSFET power switches, is typically employed where a specific set of operating conditions and part mounting choices may cause the device to exceed its maximum operating temperature. One typical protection method involves measuring the temperature at the center of the power device and comparing it against a reference. When the measurement shows that the equivalent temperature has exceeded a predetermined value, such as 150xc2x0 C., the device is shut down. A typical choice for measurement involves the use of a forward biased diode with a temperature coefficient of approx. xe2x88x922 mV/xc2x0 C. placed at the hottest part of the power device, typically its center. The diode voltage is compared with a predetermined threshold voltage corresponding to the absolute temperature of interest, such as 150xc2x0 C. The output of the comparator is used to trigger shutdown or other protection operations as desired.
In protection schemes employing a separate sensing device such as a diode, typically the sensing device must have both electrical isolation and barrier guard rings for noise isolation from the power MOSFET. Interconnect must be routed to the center of the power MOSFET to bias the device and to carry the measurement voltage to the comparison circuitry. Such requirements typically result in a significant breakup of the center of the power MOSFET, thus increasing its silicon area.
There are also problems that arise due to the reliance upon measuring absolute temperature. First, the measurement is subject to variability arising from semiconductor process variations. Additionally, the technique is non-predictive, i.e., a power dissipating fault must continue until the die temperature is pumped up to an absolute value that trips the detection circuit. Testing of the protection circuitry may be very difficult or impossible, because testing risks destruction of the device. It would be desirable to overcome these shortcomings of prior device protection techniques.
In accordance with the present invention, a thermal sensor is disclosed that is formed in an integrated fashion with a power-dissipating device. The thermal sensor does not rely on absolute temperature measurement, and therefore can be tested more effectively and can be used in a predictive manner.
The disclosed sensor exploits the Seebeck effect, in which a current can be made to flow in a circuit having two dissimilar-metal junctions maintained at different temperatures. This effect is obtained through a novel arrangement on a semiconductor integrated circuit using dissimilar conductors. A disclosed sensor employs aluminum and polysilicon conductor pairs, but this effect can be achieved with other pairs such as aluminum and copper.
The disclosed thermally monitored power-dissipating device includes a power dissipating device structure that generates a temperature difference between a relatively cold peripheral area of the device and a relatively warm central area of the device, wherein the temperature difference has a known relationship to electrical operating conditions of the device. In one embodiment, the structure includes two side-by-side arrays of source/drain diffusions of a power MOSFET, wherein the central area is an area between the two arrays and the peripheral area lies at the outer edge of the device.
A Seebeck effect thermoelectric sensor is integrally formed with the device structure. The sensor has one or more warm junctions at the central area of the device and one or more cold junctions at the peripheral area of the device, and generates an electrical output signal having a known relationship to the temperature difference between the peripheral and central areas of the device so as to provide an indication of the electrical operating conditions of the device. In one embodiment, the Seebeck effect sensor comprises alternating metal and polysilicon traces, wherein the polysilicon traces lie between source and drain diffusions of a power MOSFET just as do active polysilicon gates. Such a sensor is easily formed in an integrated fashion with the power MOSFET without requiring separate bias conductors or guard structures. Multiple pairs of conductors can be placed in series to obtain a higher-gain Seebeck sensor, resulting in greater sensitivity and/or noise immunity.
Other aspects, features, and advantages of the present invention will be apparent from the detailed description that follows.